The disk and the target BMS must be located in the same AZ.
The BMS must be in Running or Stopped state.
Device Type of the EVS disk must be SCSI.
A non-shared EVS disk must be in Available state.
A shared EVS disk must be in In-use or Available state.
BMSs using some flavors or images cannot have EVS disks attached because the servers do not have SDI iNICs or for other reasons.

What Are the Restrictions for Attaching a Disk to a BMS?